The present invention relates to power transmission noise suppression for high speed positive drive pulley and belt systems and in particular to a pulley design for business machines.
Positive drives as referred to in this disclosure include all flexible drive belts having a plurality of equally spaced teeth that engage a pulley that has a plurality of corresponding equally spaced teeth to transmit power from a power source to a driven device with minimum slippage and preferably little noise between the belt and the pulley.
Positive drive pulley and belt systems in the past that operated at high speed usually generated objectionable noise to the surrounding environment. The origin of the noise was a combination of factors; namely, the high rotational speed and the small motor pulley diameter that was necessary to obtain the desired speed ratios, and the initial contact of the belt teeth with the pulley teeth.
Although business machines in general are subject to noise generated by high speed positive drive systems, typewriters have been selected for purposes of emphasizing the effect of noise in a quiet office area.
With the advent of portable electric typewriters, most of which are used in quiet environments, noise became increasingly more noticeable and disturbing. The efficiency of persons who require concentration or uninterrupted conversation with others has been known to be adversely affected by extraneous sounds. Cognizant of the desire for greater noise suppression in such equipment, typewriter manufacturers have implemented costly noise research programs to reduce the noise emission from the typewriter and in particular, the drive system of the typewriter.
Improvements to reduce the noise emission from the typewriter have been made over the years, but none has ever met with complete pubic acceptance. Efforts to reduce the sound vibrations initiated by the drive systems of all business machines have been made over a long period of time.
For example, U.S. Pat. No. 2,987,932 to R. E. Szonn teaches a combination belt and gear (pulley) made of elastic material. The novelty lies in the tooth space angle of the gear in relation to the tooth or teeth of the belt when they are in mesh to be greater than the included tooth angle of the belt. Therefore, the upper portions of the belt progressively deform within the voids between the belt tooth and the tooth space of the gear when meshing with the gear to absorb the shock forces resulting in less vibration and less noise. The tooth space angle being the angle formed by opposing faces which define the gear tooth space and the tooth angle of the belt being the angle formed by respective faces of the belt tooth.
Another example of an approach to reducing the noise generated by a belt and pulley drive system is disclosed in U.S. Pat. No. 2,988,925 granted to H. C. Sauer. The patent relates to a positive drive belt with teeth having transverse grooves therein to reduce wear and noise during operation. The grooves in the belt teeth render the flanks of the individual teeth more flexible and resilient. Therefore, as the belt teeth mesh with the teeth of a pulley, the belt tooth flanks yield, thereby adapting more flexibly as they mesh with the pulley teeth, resulting in a less noise power transmission drive system. Both patents mentioned above relate to special belts having a specially designed tooth configuration.
The present invention depends entirely on the contour of the teeth and valleys therebetween, used in conjunction with a standard tooth drive belt to transmit power from one location to another without slippage between the pulley and the belt.
For the purpose of clarity, the term "valleys", as used in this disclosure, includes the distance from the base of one tooth to the base of an adjacent tooth excluding the opposing faces thereof.
The manufacture of toothed drive belts is a complex molding procedure. In order to give the belt the necessary strength and elasticity to perform without failure over long periods of time a variety of combinations of construction materials are required. Equally important as the combination of materials used in the belt construction, is the expertise of the personnel responsible for the molding process. In other words, the success or failure of the molded belt during operation depends largely on the molding knowledge of the mold operator at the time of manufacture. Furthermore, as various new combinations of materials are used to meet more stringent requirements such as wear and flexibility, it is often necessary to alter the molding techniques in order that the various materials be properly bonded together to achieve these requirements. It is situations such as this that the expertise of the mold operator is paramount.
Additionally, if the materials are not compatible during the process bonding, the mold operator, utilizing his years of experience in molding techniques, will be able to ascertain what appropriate adjustments to the molding process are required to insure proper material bonding compatibility. It can be seen from the foregoing that the molding of power transmission drive belts is an imperfect art at best. The success in the molding process in the manufacture of power transmission belts is determined by many criteria, all of which, when combined, result in the manufacture of a reliable and long-wearing belt. These criteria may include environmental temperature and humidity, mold temperature parameters, molding time, construction materials, and above all operator expertise.
In contrast, the molding or casting of pulleys require an expertise to a much lesser degree and having greater manufacturing tolerance parameters. For example, the materials used in the manufacture of pulleys are generally pre-formulated and require no additional care other than insertion into the molds after preheating. Elasticity is not usually a required characteristic and therefore the pulleys are dimensionally stable.
The present invention possesses advantages not available in the prior art in that standard positive drive belts may be utilized with the disclosed pulley embodiment resulting in quieter power transmission with no sacrifice in cost.
Positive drive concepts in the prior art usually rely on a special belt tooth design to reduce noise. This approach to the noise problem is significantly more costly and generally less successful than the special pulley tooth contour of the present invention.